Ground water assessment and the design of basement waterproofing

Abstract

This paper discusses the importance of groundwater risk assessment in basement waterproofing design. It discusses why it is required to allow the design of a cost-effective waterproofing system and also to determine whether the simple design approach in BS8485 can be used to assess risk from ground gases. It is also an important part of assessing the risk of vapour intrusion into basements from hydrocarbon contamination. External drainage for basements can be designed to be an effective and durable solution that lowers the hydrostatic pressure on the waterproofing system. It does require discharge to an outfall, however, and discharge to sewers is not appropriate, because water companies will not allow groundwater to be drained into public sewers. The rate of groundwater flow may also exceed the allowable discharge rates that are used in sustainable drainage (SuDS) design. This limits where external drainage can be provided to locations where a soakaway or a watercourse is available for discharge. The same constraints should also apply to cavity drain discharges. Where a basement is located in hydrocarbon contaminated groundwater or ground gas is present (such as methane or carbon dioxide) it is important to complete an appropriate risk assessment that considers whether flow into the basement will be via gas/vapour phase migration above the groundwater or dissolved phase below it. The simple screening and points system in BS8485 is not suitable where basements are below the groundwater table. The durability of materials (membranes, cavity drains, etc.) in contact with contaminants also needs consideration. Where contamination such as dissolved methane is present, cavity drains will require intrinsically safe pumps. Groundwater and contamination intrusion assessments are best completed by qualified ground engineering professionals and SoBRA accredited risk assessors for permanent gas and/or vapour intrusion respectively.